Separating suspended solids from an impure influent by addition of a flocculating agent to obtain a clear liquor overflow discharge is conventional. More particularly, it is common practice to supply an impure influent feed to a separator tank. During the separation process, a clear liquid overflow is withdrawn from the top of the tank and a concentrated slurry containing separated solids is withdrawn from the bottom as an underflow. To effect a clear overflow, a flocculating agent is added to the feed to facilitate flocculation and precipitation of suspended particles. Consequently, impure influent is separated into a large volume of clear overflow and a small volume of underflow having a high solids content.
A sedimentation device within the purview of the subject invention comprises a large diameter cylindrical vessel with a vertical axis into which a turbid liquid feed is deposited. A fast throughput type separator is particularly useful in the practice of the subject invention. That separator is designed so that a pretreated contaminated influent, an influent to which is added a flocculating agent, is introduced at a controlled velocity in a horizontal direction in an active sludge bed within the separator. That introduction results in the elimination of a free settling zone. Particles move randomly through the sludge bed in the separator, promoting additional agglomeration, which results in accelerated settling. Rakes aid the compaction of settled solids while moving them to the discharge area. Use of such separators advantageously results in a sharp interface between the sludge bed and clarified effluent.
This separator operates in a manner similar to those previously described. Influent is introduced into the center of the unit. A vertical feed pipe extends into the unit and is faced by a baffle plate forcing influent to enter in a horizontal direction. The gap between the baffle plate and the end of the feed pipe determines the velocity with which the feed is introduced into the sludge bed. Alternatively, side inlet or bottom inlet units can be used to feed from above or below the separator. Solids collect in the bottom of the unit where rotating rakes move them to a centrally located outlet from which they are discharged. A clarified overflow is discharged from the top over a circumferential overflow weir.
The necessity for pretreatment depends on the chemical characteristics of the solid concentration, particle size and particle characteristics of the influent. In most applications, conditioning of the feed consists of simply adding an agglomerating or flocculating agent to the feed. Deaeration, pH adjustment, addition of a second agglomerating agent, and variation of reaction times are also available for pretreating the influent.
Use of multiple sedimentation devices is also within the purview of the subject invention. A countercurrent decantation technique can be employed in the operation of multiple units. Simply, this technique utilizes addition of wash solution to the last separator and advancing the resulting overflow upstream countercurrent, that is opposite, to the flow of underflow solids. Specifically, a liquid-solid feed is introduced at one end of a line of separators, and a clear weak wash solution is introduced at the other end. The solids go from the bottom of one separator to the top of the next toward the separator where the wash solution enters. A clear solution overflows from one separator to the next but in a direction opposite to the movement of the solids. This results in a very dense underflow being discharged from the last separator. Thus, an overflow product issues from the first separator; a solid residue underflow issues from the last separator. The main idea is to get the sludge as thick as possible while keeping the overflow clear.
For efficient treatment of influent, mass flow rate into and out from the separator should be maintained at constant rates to remove the bulk of the solids from the influent while permitting a clear overflow. Further, the feed rate of solids must balance the solids discharge rate.
Conventionally, feed rates were controlled manually using visual observation of the underflow and the overflow. Adjustments based on such observations resulted in a see-saw operation to maintain an essentially balanced control over the separator. Such an operation is particularly troublesome in countercurrent decantation operations. Thus, accurate control of the feed rate into and out of the separator is important to maintain steady state conditions. Moreover, for manual operation to meaningfully minimize disruptive effects produced by fluctuations of numerous process variables, such operation would necessitate use of an excessive number of operators. This approach is commercially prohibitive in view of the labor expenses involved. Furthermore, lack of continuity in adjustments to correct for fluctuations of process variables tends to upset optimum operating conditions for the sedimentation operation. Also, the rather erratic and unpredictable occurrences of such disruptive factors in the process upset the stabilization of the operation.
In addition to the enumerated shortcomings generally associated with manual operation, other drawbacks exist. Practically, manual operators attempting to achieve stabilization often overcompensate for disruptive effects. Such overcompensation tends to produce additional disruptions or surges throughout sedimentation systems using multiple separators, thereby attenuating disruptions to steady state operation. Consequently, an efficient and trouble-free operation is commercially impossible to achieve with manual operation.
In the mining industry, control of a separation process can be critical for efficient recovery of valuable metals like copper, molybdenum and uranium. Loss of such metals to tailings is uneconomical. Consequently, advances in the art of liquid-solid separation involving control of separator operation has commercial significance.
U.S. Pat. No. 3,208,592 to Smith discloses a method for controlling hydroseparators. Such a separator is a thickener having a means for introducing a backwash fluid near an underflow discharge. A control system is taught which coordinates overflow and underflow discharges. Specifically, an influent and wash liquid are continuously introduced into the thickener. A constant density of discharging underflow solids is maintained by controlling the volume discharged. Overflow rate is maintained constant by controlling introduction of the wash liquid. Because the density of the underflow discharge is maintained constant and because the amount of backwash liquid varies proportionally with the flow rate of the discharge, the flow rate of backwash liquid is controlled by maintaining that rate in a constant ratio to the flow rate of the discharge.